The present invention relates to computed tomography machines that are free-standing or that are part of a radiation therapy machine, and in particular to a computed tomography machine providing a reduced dose to the patient.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope or a megavoltage energy x-ray source, such as a linear accelerator. The external source produces a collimated radiation beam directed along an axis of radiation toward a tumor site. The adverse effect of irradiating healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the radiation beam into the patient along a variety of radiation axes with the beams converging on the tumor site.
Intensity modulated radiation therapy (IMRT) provides an external beam of radiation composed of individually intensity-modulated “beamlets”. The intensities or weights of the beamlets are calculated to provide a desired dose pattern of radiation for an arbitrary shaped tumor within the patient while minimizing radiation in other areas. One such system, commercially available from TomoTherapy, Inc. of Madison, Wis., provides a radiation source that rotates about the patient on a helical line producing a fan beam of multiple beamlets, each beamlet controlled by a movable shutter.
With the increased accuracy possible with IMRT, the problem arises of obtaining precise knowledge of the position, shape, and size of the target which often changes between the time that the treatment plan was developed and the actual treatment time. In image guided radiation therapy (IGRT), the target geometry is monitored at the time of radiation treatment so that the treatment plan may be adjusted accordingly. Ideally, the imaging system used for this purpose is computer tomography (CT) which can provide precise three-dimensional imaging of a target needed for maximizing the benefit of IMRT.
One method of providing IGRT with CT is to provide a rail system to transfer the patient between an adjacent CT and the IMRT machines. Alternatively, a CT machine may be incorporated into the IMRT machine itself, either by adding a kilovoltage x-ray source and detector, or by using the radiation therapy megavoltage source and adding megavoltage detector.
Generally, this latter approach of megavoltage CT (MVCT) requires high imaging doses because of the penetrating nature of megavoltage beams, low detector efficiency and the need to repeat imaging on a daily basis for patient positioning. A common strategy to reduce the exposure to the patient in MVCT is to reduce the area of the imaging, for example, by limiting radiation principally to the treatment area, this latter option making dual use of the radiation for both treatment and imaging. Unfortunately, reduction of the exposure area in CT creates “partial volume” artifacts in the reconstructed image. These artifacts are caused by structure outside of the image area which affects the measurements of the image region to the extent that radiation passes through this outside structure to reach the image area. Because the outside structure is not fully characterized by imaging, its attenuating influence is not fully cancelled, and the result is streaks in the image.
U.S. Pat. No. 6,618,467 issued Sep. 9, 2003, assigned to the assignee of the present invention, and hereby incorporated by reference, describes an MVCT system that addresses this problem of partial volume artifacts by performing a pre-scan of an entire patient slice with low flux radiation to obtain a low resolution measurement of the outside structure. This low flux scan is combined with the radiation used during the treatment to provide images with reduced partial volume artifacts.
Alternative methods of dealing with the partial volume artifacts are described for example, in U.S. Pat. No. 4,878,169 issued Oct. 31, 1989, and U.S. Pat. No. 6,810,102 issued Oct. 26, 2004, which also augment CT projection data taken of a limited region of interest with full slice projection data obtained at a different time.